Due to the very nature of their operation, power system transformers must manage high voltage levels. For example, distribution transformers commonly reduce transmission voltages from distribution levels of 2.3 kV to 69 kV down to final utilization levels of 120V to 480V. In so doing, power system transformers use high voltage bushings to input the distribution level voltages. The high voltage bushings are attached to the primary windings of the transformer. Because the transformer windings reside in oil-filled tanks, the high voltage bushings commonly are affixed to the outside of oil-filled transformer tanks. The high voltage bushings are then connected to the primary windings through holes in the transformer tank. In the case of pad-mounted transformers, the oil-filled tanks and high voltage bushings are further surrounded by an outside enclosure for safety reasons.
In order to protect the transformer and protect the public from electrical hazards, the high voltage bushings are sized in accordance with strict industry standards. Accordingly, the corresponding holes made in the transformer tanks must be within strict tolerances. If the holes are out of tolerance, the standard sized high voltage bushings may be too large for the holes, rendering the tank unusable. Alternatively, the holes may be too large for the high voltage bushings, permitting water damage to the transformer and safety concerns for the public.
Manufacturing a pad-mounted transformer tank begins by making holes for the high voltage bushings in flat sheets of metal. The flat metal sheets eventually are formed into rectangular enclosures. The process of making holes for the high voltage bushings in pad-mounted distribution transformers is further complicated by an industry-accepted standard that requires each hole to have a circular raised base, formed at a twelve degree angle with the enclosure. In other words, before punching a hole in the metal sheet, the sheet must have a circular deformation that forms a twelve-degree angle with the rest of the metal sheet. Offsetting the high voltage bushing at twelve degrees permits easier installation and allows the high voltage bushing to use less space in the entire pad-mounted transformer enclosure.
A process called embossing is used to make the circular, raised deformation. Embossing is a process by which metal is lifted and deformed. Either before or after the metal sheet is embossed, a hole is punched such that the embossed portion forms a circular base around the punched hole. FIGS. 1A and 1B show one example of a prior art embossing and hole punching process 100. In FIG. 1A, an embossing machine 101 embosses a metal sheet 104 by deforming 105 one side of metal sheet 104 with an embossing punch 103, without causing, a hole. In order to meet the requirements for a high voltage bushing (not shown), embossing punch 103 has an angled punch head 107 at an angle .alpha.. Angled punch head 107 creates an angle .beta. in metal sheet 104. In accordance with industry standards for high voltage bushings, angles .alpha. and .beta. commonly are set at twelve degrees.
An operator then removes embossed metal sheet 104 from embossing machine 101 and places it in a hole-punching machine 110. As shown in FIG. 1B, hole-punching machine 110 has a hole punch 102 with a flat punch head 108. Hole punch 102 also has a radius r2 that is less than a radius r1 of embossing punch 103. The smaller radius r2 forms a hole 106 with an embossed base raised at an angle of twelve degrees, as required in the transformer industry.
In order to punch a sufficiently round hole, the operator must rotate metal sheet 104 twelve degrees in a counterclockwise direction. The twelve-degree rotation permits hole-punching machine 110 to make a sufficiently circular hole to meet the strict industry standards. In addition, in order to ensure that a precision hole is formed, embossed portion 105 must be placed directly under hole punch 102. Thus, in order to keep the radius of punched hole 106 within the specified tolerance, the machine operator must carefully align hole punch 102 over deformation 105 made by the embossing punch 103. The further hole punch 102 is out of alignment with the deformation 105, the more out of tolerance hole 106 will be. However, visually aligning deformation 105 with hole punch 102 is a difficult and imprecise process. Moreover, once the operator aligns hole punch 102, metal sheet 104 may move while the operator is securing it to hole punch machine 110. Accordingly, it often takes many attempts and many unusable metal sheets to create a hole within the strict tolerance required for transformer enclosures. Although there are laser-cutting devices that can create holes within the required tolerance, these devices are far more complex and more costly.
Therefore, it would be advantageous to provide a system and method that would remove the need for human intervention and eliminate the error inherent in aligning an embossed metal sheet with a hole punch. In addition, by removing human intervention, the embossing and punching process may be automated using robots. However, current robotic technology is incapable of rotating metal sheet 104 twelve degrees in a counterclockwise direction as required in prior art process 100. Automating this process may also contribute to the robotic automation of an entire manufacturing or assembly line process.